Plasma Plumes Shield Earth From Solar Storms

The magnetosphere, Earth’s magnetic field, stretches from the core of the planet out into space. Here, the magnetosphere meets the solar wind, a stream of charged particles emitted by the sun. The magnetosphere, for the main part, acts to shield the Earth from this high-energy solar activity.

When the Earth’s magnetosphere comes into contact with the sun’s magnetic field in a process called “magnetic reconnection,” the Earth’s atmosphere can be invaded by powerful electrical currents from the Sun. These currents whip up geomagnetic storms and space weather phenomena that can affect everything from high-altitude aircraft to the astronauts aboard the International Space Station (ISS).

A new study from MIT and NASA, published in a recent issue of Science, has identified a process in Earth’s magnetosphere that reinforces the shielding effect and keeps incoming solar energy at bay.

The team combined observations from both ground and space to examine a plume of low-energy plasma particles that basically hitch a ride along magnetic field lines. These field lines stream from Earth’s lower atmosphere up to the point where the planet’s magnetic field connects with that of the sun, tens of thousands of kilometers above the surface. The scientists call this region the “merging point.” It is here that the presence of cold, dense plasma slows magnetic reconnection, blunting the sun’s effects on Earth.

“The Earth’s magnetic field protects life on the surface from the full impact of these solar outbursts,” says John Foster, associate director of MIT’s Haystack Observatory. “Reconnection strips away some of our magnetic shield and lets energy leak in, giving us large, violent storms. These plasmas get pulled into space and slow down the reconnection process, so the impact of the sun on the Earth is less violent.”

The researchers at Haystack Observatory have spent more than a decade studying plasma plume phenomena using a ground-based technique called GPS-TEC, in which scientists analyze radio signals transmitted from GPS satellites to more than 1,000 receivers on the ground. Incoming radio waves can be altered by large space-weather events, such as geomagnetic storms. Scientists are able to use this distortion to determine the concentration of plasma particles in the upper atmosphere. This data can then be used to produce two-dimensional global maps of atmospheric phenomena, such as plasma plumes.

Key characteristics of the plasma plumes have been illuminated by these ground-based observations, such as how often they occur, and what makes some plumes stronger than others. The two-dimensional mapping technique, however, only gives an estimate of what space weather might look like in low-altitude regions of the magnetosphere. Direct observations from space are needed to obtain a more precise, 3D picture of the entire magnetosphere, Foster said.

Foster enlisted the help of Brian Walsh from NASA’s Goddard Space Flight Center. Foster brought Walsh data showing a plasma plume emanating from the Earth’s surface, and extending up into the lower layers of the magnetosphere, during a moderate solar storm in January 2013. Walsh compared this data with the orbital trajectories of three spacecraft that have been orbiting Earth to study auroras in the atmosphere.

They found that all three spacecraft have crossed the point in the magnetosphere at which Foster had detected a plasma plume from the ground. The data from each spacecraft was analyzed and the team found that the same cold, dense plasma plume stretched all the way up to where the solar storm made contact with Earth’s magnetic field.

The space observations validate the ground-based measurements, according to Foster. The combination of space- and ground-based data also gives a highly detailed picture of a natural defensive mechanism in the planet’s magnetosphere.

“This higher-density, cold plasma changes about every plasma physics process it comes in contact with,” Foster says. “It slows down reconnection, and it can contribute to the generation of waves that, in turn, accelerate particles in other parts of the magnetosphere. So it’s a recirculation process, and really fascinating.”

Foster compares this plume phenomenon to a “river of particles,” saying it is not unlike the Gulf Stream, which is a powerful ocean current that influences the temperature and other properties of surrounding waters. The plasma particles can behave the same way, on an atmospheric scale, by redistributing throughout the atmosphere to form plumes that “flow through a huge circulation system, with a lot of different consequences.”

“What these types of studies are showing is just how dynamic this entire system is,” Foster added.